The present invention is directed to a twist drill or rock drill with an axially elongated drill shank having grooves in its outer surface for conveying drillings and with a hard metal drill head at one end of the shank.
It is well known that rock drills wear out very rapidly in the drill head region when drilling hard rock and using high rpms. Accordingly, such rock drills are formed of hard metal, and in the present case the term "hard metal" includes sintered or fused carbides, silicides, borides or their alloys.
Such a rock drill is disclosed in DE-AS 20 08 825. This rock drill has an elliptically shaped drill head formed of hard metal and soldered to the drill shank and cooperates with the shank by means of a mortise joint with the joint being formed by a tenon-shaped extension of the drill head soldered in a bore in the drill shank appropriately dimensioned in its diameter and depth.
In such a rock drill a continuous removal of the drilling takes place between intermediate spaces formed between the smallest diameter of the elliptical drill head and the inside wall of the borehole. There are two such intermediate spaces which become smaller in the region of the largest diameter of the drill head. Drillings conveyed through the intermediate spaces tend to jam in the narrowing section of the intermediate spaces located diametrically opposite one another, so that premature wear or damage to the drill head occurs in the circumferential region of the largest diameter. Apart from premature mechanical wear of the drill head, the drilling gap between the circumference of the drill head and the borehole wall causes an enlargement at the bore and a rough borehole surface.